Wire-protective tubes for automotive wire harnesses serve to bind coated electric wires and to protect the wires against the impact, friction, and abrasion caused by contacts with other automotive parts. These tubes have hitherto been molded mainly from non rigid vinyl chloride polymer (hereinafter referred to also as PVC). However, halogen-free materials are coming to be employed in place of poly(vinyl chloride) for producing automotive wire harness parts, e.g., tubes, so as to cope with recent measures for the preservation of global environment. Such halogen-free materials heretofore in use are compositions which comprise an olefin elastomer, e.g., a propylene/(ethylene/propylene) copolymer, and a metal hydroxide added thereto as a flame retardant so as to give importance to heat resistance and abrasion resistance.
On the other hand, wire-protective tubes for automotive wire harnesses are recently coming to be increasingly required to have higher flame retardancy. However, it is becoming impossible, with the compositions comprising an olefin elastomer and a metal hydroxide, to attain the flame retardancy required. Furthermore, it is advocated as a part of measures for environmental preservation to reduce fuel consumption by reducing the weights of motor vehicles. For attaining this, weight reduction in parts including wire harnesses also is important. However, poly (vinyl chloride) and the compositions comprising an olefin elastomer and a metal hydroxide have a specific gravity of 1.3 or higher and this is an obstacle to weight reduction.
An olefin elastomer composition comprising an olefin elastomer, e.g., a propylene/(propylene/ethylene) copolymer, and a bromine compound flame retardant incorporated therein is reported in JP-A-2000-290439 for the purpose of attaining a high degree of flame retardancy and weight reduction.
The wire harnesses disposed in automotive engine rooms are always exposed to high temperatures, and there are cases where the ambient temperature exceeds 150° C. However, use of wire-protective tubes made of those olefin elastomer compositions in this application poses a serious problem that the wire-protective tubes, in which the olefin elastomer is the only resin ingredient, soften to become unable to retain their shape, i.e., are deprived of their function as a protective material. In the case of the olefin elastomer composition containing a propylene/(propylene/ethylene) copolymer as the only resin ingredient, there are problems that appearance failures are apt to occur due to melt fracture during tube molding, that insertion of coated wires into the tube is difficult, and that wire tips may mar the inner surface of the tube. In addition, among olefin elastomers, ethylene/propylene copolymers, for example, have a serious problem that, as compared with propylene/(propylene/ethylene) copolymers, impartation of flame retardancy thereto is difficult in olefin elastomer compositions containing a bromine compound flame retardant and a high degree of flame retardancy cannot be imparted especially in tubes, which are thin-wall moldings.
With the desire for improvements in flame retardancy, a problem has arisen that the flame retardants incorporated cause appearance failures through blooming, etc. For example, tetrabromobisphenol A bis(2,3-dibromopropyl) ether is known as a flame retardant which imparts high flame retardancy to polyolefins. However, impartation of a flame retardancy of oxygen concentration index 24, which is required of wire-protective tubes, with this bisphenol A derivative results in enhanced blooming, although satisfactory heat resistance is obtained. Tetrabromobisphenol S bis (2,3-dibromopropyl) ether is known as a flame retardant which also imparts high flame retardancy to polyolefins and which is reduced in blooming. However, when used with a polymeric material having a high melt viscosity, this flame retardant decomposes during extrusion molding to cause a molding failure, i.e., burn marks. Tris(2,3-dibromopropyl) isocyanurate also is known as a flame retardant which imparts high flame retardancy to polyolefins. However, when this flame retardant is used in such an amount as to impart a flame retardancy of oxygen concentration index 24, part of the flame retardant adheres to the sizing die and roll and then readheres to the surface of the wire-protective tube to cause an appearance failure.